1. Field of the Invention
The present invention relates to a negative electrode for a lithium rechargeable battery and the lithium rechargeable battery adopting the same, and more particularly to, a negative electrode for a lithium rechargeable battery that uses a complex material of metal or non-metal and oxide thereof and a carbonaceous material as a negative electrode active material, and the lithium rechargeable battery adopting the negative electrode.
2. Description of the Related Art
Portable electronic devices such as personal digital assistants (PDAs), mobile phones, notebook computers, digital cameras, and the like, portable electronic devices have become very popular and are increasingly smaller in size and lighter in weight. This has resulted in a strong demand for a battery suitable for a power source of portable electronic devices. In particular, among rechargeable batteries that can be charged and discharged, a lithium rechargeable battery, having a lower discharge rate and a higher energy density than conventional lead battery, nickel cadmium battery, etc., is widely used for high electronic devices.
Although a lithium metal having a high energy density has been suggested as a negative active material of the lithium rechargeable battery, the lithium metal has a problem with its safety since a dendrite in a negative electrode can form when the lithium rechargeable battery is charged. The dendrite can extend through a separator between electrodes and potentially cause an internal short circuit.
Further, the dendrite can have a very large surface area with a high reactivity, and can form a high-polymer capture site having a low electronic conductivity on the surface of the negative electrode by reacting to electrolyte. This can cause the resistance of a battery to rapidly increase, such that particles are isolated from an electronic conductivity network, which inhibits the battery from being charged and discharged.
Due to the above disadvantages of the battery using the lithium metal, a graphite material capable of absorbing and exhausting lithium ions instead of the lithium metal has been used as the negative electrode active material. When graphite is used as the negative electrode active material, metal lithium is not precipitated, which does not causes the above problem with the dendrite. However, the theoretical discharge capacity of the lithium metal comes to 3860 mAh/g when the lithium metal is employed as the negative electrode, but the theoretical discharge capacity of the graphite comes to only 372 mAh/g when the graphite is employed as the negative electrode. Hence, a new active material having a higher capacity than the graphite active material is needed.
In order to increase a discharge capacity of a battery, metal materials such as Tin (Sn), Aluminum (Al) or Zinc (Zn), and the like are formed into a lithium compound and non-metal materials such as Si, Germanium (Ge), Boron (B), Phosphorus (P), and the like, or oxide thereof have been suggested as negative electrode active materials. Although active materials of the metal, non-metal materials, or oxide thereof have a theoretically higher capacity than graphite, they have a high capacity at an initial stage, but since these active materials have a low electrochemical reversibility, they are often disadvantageous in that capacity falls quickly during a charge/discharge cycle. This disadvantage causes a reduction in the lifetime of a battery. To address this problem, it has been suggested to use an active material (hereinafter referred to as metal-carbon combination active material) in combination of the material forming the lithium compound and a carbonaceous material.
The metal-carbon combination active material can be manufactured by burying the metal and non-metal materials, or the oxide particles thereof, in a carbonaceous material or by coating the metal and non-metal materials with carbon and then burying them in the carbonaceous material, and by mixing the metal and non-metal materials with the carbonaceous material at a high temperature.
A negative electrode of a lithium ion battery comprises an active material that joins the battery reaction, a current collector, a high-polymer binder that combines the active material and the current collector and adheres the active material to the current collector. The active material used for the negative electrode of the lithium ion battery is in the form of particles and is adhered to the current collector by the high-polymer binder. Particles of the active material adhered by the high-polymer binder are electrically connected to each other and are coupled to the current collector by point contact. Therefore, if particles of the active material have a low degree of the point contact, i.e., if particles of the active material have a small point contact area, a battery internal resistance has a large value, and a particle of the active material that is isolated due to the small point contact does not contribute to a battery capacity. Hence, it is important to maintain a large contact area between particles of the active material.
A battery reaction between positive and negative electrodes of a battery occurs when the battery is charged and discharged. If the battery reaction occurs, the lithium ion is inserted into or exhausted from an active material particle structure of the negative electrode, resulting in an expansion or shrinkage of particles of the active material. Although it depends on a type of a material used as the active material, natural graphite has a bulk variation of 10% at the maximum, and the metal-carbon combination active material has a bulk variation greater than or equal to 10%. Therefore, an electrical connection between particles of the active material by point contact may be unstable during charging and discharging.
Due to the bulk variation and instability between particles of the active material during charging and discharging, internal resistance of the lithium ion battery, can gradually increase and a battery capacity can gradually decrease whenever the lithium ion battery is charged and discharged, resulting in a reduction of the lifetime of the lithium ion battery. Although a lifetime degradation caused by the bulk variation of the active material can be reduced by adding a material such as carbon black as a conductive material to the lithium ion battery, an excessive use of this conductive material can cause a reduction in a component ratio of the active material, which reduces a discharge capacity of the lithium ion battery.